Solving the "No-Win" Scenario: Mercer Island, Washington Sewer Lakeline Replacement

During the cleanup of Lake Washington in the 1950s and 1960s, shorelines of lakeside communities proved to be convenient locations for new sewers. Over 50 years later, many of these facilities are reaching the end of their design life and require complex replacements. Replacement of Reach 3 of the City of Mercer Island's lakeline sewer presented examples of the hard decisions that regional sewerage agencies face in the coming years. Reach 3 consisted of two low-pressure pump stations and about 2 miles of sewer offshore in Lake Washington. The primarily asbestos-cement system had been experiencing physical deterioration, capacity deficiencies and operational problems. After years of evaluating alternatives and coordinating with stakeholders, the City and Tetra Tech implemented an in-lake replacement. The low-pressure system was replaced with a gravity-flow and pumped system that maintains self-cleaning velocities. The project included 20,000 feet of new 6-inch to 16-inch lakeline and a new onshore pump station over 50 feet deep.